Both a Subscriber Identity Module (SIM) card and a Universal SIM (USIM) card belong to intelligent cards, which are certificates for a user to log in; that is, an SIM/USIM card is used for uniquely identifying a subscriber. For a terminal user, the SIM/USIM card has the following four functions: 1) a user data storage function, for example, storage of International Mobile Subscriber Identity (IMSI) and an authentication Key (K); 2) a user security management function, for example, operation and management of a Personal Identification Number (PIN), a PIN Unlocking Key (PUK) and so on; 3) a user identity authentication function, that is, an authentication algorithm and an authentication result are provided to identify a legal identity of the user, so that the user can access a network successfully; 4) an operator customized service function, for example, operator customized valued-added services like USIM card Application Tool Kit (USAT).
Because the SIM card is a certificate for the user to log in, when the terminal user roams from an area A to an area B, making a call, accessing the Internet and conducting data services in the area B will incur high roaming charges. At the same time, because the user is in a roaming state, and has a relatively low priority of an operator system in the area B, neither network speed nor call quality can be fully ensured.
For solving the above problem, the terminal user generally changes the SIM card to implement a normal conversation in a roaming area, so as to enjoy a lower price and enjoy the same service as a local card.